P6. Abstract The accessory protein Vpr facilitates HIV-1 replication in dividing T cells, ensuring orderly progression through the HIV-1 life cycle. The most remarkable Vpr phenotype is induction of DNA damage checkpoint and cell cycle arrest in G2/M phase, although the underlying details are the still elusive. There is compelling evidence that Vpr interacts with several cellular proteins that recognize damaged DNA, targeting them for proteasomal degradation via CRL4DCAF1 E3 ubiquitin ligase. While evident that, in principle, Vpr antagonism of DNA repair ultimately can be of benefit to HIV-1, the full extent of Vpr-mediated subversion of cellular DNA repair is not known. In this project, we propose to systematically explore the extent of Vpr's engagement with the DNA repair machinery, focusing on DNA repair pathways that recognize and process ?marks of damage?, introduced into HIV-1 cDNA during reverse transcription. We aim to discover and validate other specific DNA repair protein(s) that are targeted by Vpr, in particular those mediating the induction of DNA damage checkpoint and cell cycle arrest. Further, we will biochemically, biophysically and structurally analyze the interactions between Vpr and the already identified targets HLTF, MUS81-EME1, and hHR23A, as well as any new target(s) discovered in this project. Overall, our studies will define the extent of Vpr-induced changes in cellular DNA repair, identify specific DNA repair pathways/proteins targeted by Vpr, and structurally characterize the responsible CRL4DCAF1 E3/Vpr/target complexes.